Windowed sequencing

ABSTRACT

In one implementation, a method is described. The method includes determining an operational characteristic of sensors of a sensor array. The method further includes selecting a group of sensors in the array based on the operational characteristic of sensors in the group. The method further includes enabling readout of the sensors in the selected group. The method further includes receiving output signals from the enabled sensors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 16/153,898 filed Oct. 8, 2018, which is a division of U.S. patent application Ser. No. 15/043,296 filed Feb. 12, 2016, now U.S. Pat. No. 10,100,357, which is a continuation of U.S. patent application Ser. No. 13/891,023 filed May 9, 2013, now abandoned, which disclosures are herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

This disclosure, in general, relates to methods for nucleic acid sequencing.

BACKGROUND

A variety of types of chemical sensors have been used in the detection of chemical processes. One type is a chemically-sensitive field effect transistor (chemFET). A chemFET includes a source and a drain separated by a channel region, and a chemically sensitive area coupled to the channel region. The operation of the chemFET is based on the modulation of channel conductance, caused by changes in charge at the sensitive area due to a chemical reaction occurring nearby. The modulation of the channel conductance changes the threshold voltage of the chemFET, which can be measured to detect and/or determine characteristics of the chemical reaction. The threshold voltage may for example be measured by applying appropriate bias voltages to the source and drain, and measuring a resulting current flowing through the chemFET. As another example, the threshold voltage may be measured by driving a known current through the chemFET, and measuring a resulting voltage at the source or drain.

An ion-sensitive field effect transistor (ISFET) is a type of chemFET that includes an ion-sensitive layer at the sensitive area. The presence of ions in an analyte solution alters the surface potential at the interface between the ion-sensitive layer and the analyte solution, due to the protonation or deprotonation of surface charge groups caused by the ions present in the analyte solution. The change in surface potential at the sensitive area of the ISFET affects the threshold voltage of the device, which can be measured to indicate the presence and/or concentration of ions within the solution.

Arrays of ISFETs may be used for monitoring chemical reactions, such as DNA sequencing reactions, based on the detection of ions present, generated, or used during the reactions. See, for example, U.S. Pat. No. 7,948,015 to Rothberg et al., which is incorporated by reference herein in its entirety. More generally, large arrays of chemFETs or other types of chemical sensors may be employed to detect and measure static and/or dynamic amounts or concentrations of a variety of analytes (e.g. hydrogen ions, other ions, compounds, etc.) in a variety of processes. The processes may for example be biological or chemical reactions, cell or tissue cultures or monitoring neural activity, nucleic acid sequencing, etc.

An issue that arises in the operation of large scale chemical sensor arrays is the susceptibility of the sensor output signals to noise. Specifically, the noise affects the accuracy of the downstream signal processing used to determine the characteristics of the chemical and/or biological process being detected by the sensors.

It is therefore desirable to provide methods for reducing noise in output signals of chemical sensors and improving signal to noise ratio and readout of chemical sensors.

SUMMARY

In one implementation, a method is described. The method includes determining an operational characteristic of sensors of a sensor array. The method further includes selecting a group of sensors in the array based on the operational characteristic of sensors in the group. The method further includes enabling readout of the sensors in the selected group. The method further includes receiving output signals from the enabled sensors, the output signals indicating chemical reactions occurring proximate to the sensors of the sensor array.

In one embodiment, the operational characteristic of sensors of a sensor array is selected from the group of a bead loading quality of the sensors of the sensor array, a noise spectrum of the sensors of the sensor array, and a threshold voltage value of the sensors of the sensor array. In another embodiment, readout of remaining sensors of the sensor array is bypassed. According to another embodiment, the selecting a group of sensors in the array is based on more than one operational characteristic of sensors in the group. In a further embodiment, the sensors in the sensor array include chemically-sensitive field effect transistors. According to once embodiment, the chemically-sensitive field effect transistors are arranged in rows and columns and the selecting includes selecting contiguous rows of chemically-sensitive field effect transistors in the sensor array. In another embodiment, the output signals further indicate an ion concentration due to sequencing reactions occurring proximate to the chemically-sensitive field effect transistors. According to one embodiment, the output signals are analog signals and the method further includes converting the output signals into digital signals and the receiving output signals further includes receiving the converted digital signals.

In another implementation, a method for nucleic acid sequencing is described. The method includes providing template nucleic acids to at least some of a plurality of locations coupled to sensors of an array. The method further includes analyzing output signals of the sensors of the array to identify which locations in the plurality of locations contain the disposed template nucleic acids. The method further includes selecting a group of sensors coupled to identified locations containing the disposed template nucleic acids. The method further includes introducing known nucleotides within at least some of the plurality of locations. The method further includes measuring the output signals of the selected sensors to detect sequencing reaction byproducts resulting from incorporation of the introduced known nucleotides into one of more primers hybridized to at least one of the disposed template nucleic acids.

In one embodiment, the method further comprises enabling readout of the sensors in the selected group, and bypassing readout of remaining sensors of the sensor array. In another embodiment, the sequencing reaction byproducts comprise hydrogen ions. In yet another embodiment, the sequencing reaction byproducts resulting from incorporation are of chemically similar composition for each of the known nucleotides. In one embodiment, the method further comprises determining at least a portion of sequences of at least a portion of the template nucleic acids based on the introduced known nucleotides and further based on the measured output signals. According to one embodiment, the sensors comprise field-effect transistors having a chemically sensitive portion responsive to the sequencing reaction byproducts and disposed in proximity to the locations such that the at least one of the sequencing reaction byproducts diffuse or contact the sensors to thereby be detected. According to another embodiment, the chemically sensitive portion of the field-effect transistors of the array is responsive to a plurality of different sequencing reaction byproducts. In yet another embodiment, the locations are within respective reaction chambers. In one embodiment, the measured output signals are analog signals and the method further includes converting the output signals into digital signals and the receiving output signals further includes receiving the converted digital signals.

Particular aspects of one more implementations of the subject matter described in this specification are set forth in the drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of components of a system for nucleic acid sequencing according to an exemplary embodiment.

FIG. 2 illustrates a cross-sectional view of a portion of the integrated circuit device and flow cell according to an exemplary embodiment.

FIG. 3 illustrates a cross-sectional view of representative chemical sensors and corresponding reaction regions according to an exemplary embodiment.

FIG. 4 illustrates a block diagram of an exemplary chemical sensor array of coupled to an array controller, according to an exemplary embodiment.

FIG. 5 illustrates a method, according to an exemplary embodiment.

FIG. 6 illustrates a method for nucleic acid sequencing, according to an exemplary embodiment.

FIG. 7 illustrates examples of two different groups of sensors in an array that have been selected based on an operational characteristic of sensors in the group, according to an exemplary embodiment.

DETAILED DESCRIPTION

Methods for reducing noise in output signals of chemical sensors and improving readout of output signal of chemical sensors based on the operational characteristic of the chemical sensors are described. For example, an integrated circuit may comprise an array of chemically sensitive sensors arranged in rows and columns. Output signals from the sensors indicating chemical reactions occurring proximate to the sensors of the sensor array may be read out. Determining an operational characteristic of sensors of a sensor array before the chemical reactions occur and reading out sensors based on the determined operational characteristic results in improved signal quality of output signals, for example.

FIG. 1 illustrates a block diagram of components of a system for nucleic acid sequencing according to an exemplary embodiment. The components include flow cell 101 on integrated circuit device 100, reference electrode 108, plurality of reagents 114 for sequencing, valve block 116, wash solution 110, valve 112, fluidics controller 118, lines 120/122/126, passages 104/109/111, waste container 106, array controller 124, and user interface 128. Integrated circuit device 100 includes microwell array 107 overlying a sensor array that includes chemical sensors as described herein. Flow cell 101 includes inlet 102, outlet 103, and flow chamber 105 defining a flow path for reagents over microwell array 107. Reference electrode 108 may be of any suitable type or shape, including a concentric cylinder with a fluid passage or a wire inserted into a lumen of passage 111. Reagents 114 may be driven through the fluid pathways described above, valve block 116 and valve 112, and flow cell 101 by pumps, gas pressure, or other suitable methods, and may be discarded into waste container 106 after exiting outlet 103 of flow cell 101. Fluidics controller 118 may control driving forces for reagents 114 and the operation of valve 112 and valve block 116 with suitable software. Flow cell 101 may have a variety of configurations for controlling the path and flow rate of reagents 114 over microwell array 107. Array controller 124 provides bias voltages and timing and control signals to integrated circuit device 100 for reading the chemical sensors of the sensor array. Array controller 124 also provides a reference bias voltage to reference electrode 108 to bias reagents 114 flowing over microwell array 107. Microwell array 107 includes an array of reaction regions as described herein, also referred to herein as microwells, which are operationally associated with corresponding chemical sensors in the sensor array. For example, each reaction region may be coupled to a chemical sensor suitable for detecting an analyte or reaction property of interest within that reaction region. Microwell array 107 may be integrated in integrated circuit device 100, so that microwell array 107 and the sensor array are part of a single device or chip.

During an experiment, array controller 124 collects and processes output signals from the chemical sensors of the sensor array through output ports on integrated circuit device 100 via bus 127. Array controller 124 may be a computer or other computing means. Array controller 124 may include memory for storage of data and software applications, a processor for accessing data and executing applications, and components that facilitate communication with the various components of the system in FIG. 1. The values of the output signals of the chemical sensors indicate physical and/or chemical parameters of one or more reactions taking place in the corresponding reaction regions in microwell array 107. For example, in an exemplary embodiment, the values of the output signals may be processed using the techniques disclosed in Rearick et al., U.S. patent application Ser. No. 13/339,846, filed Dec. 29, 2011, based on U.S. Prov. Pat. Appl. Nos. 61/428,743, filed Dec. 30, 2010, and 61/429,328, filed Jan. 3, 2011, and in Hubbell, U.S. patent application Ser. No. 13/339,753, filed Dec. 29, 2011, based on U.S. Prov. Pat. Appl. No 61/428,097, filed Dec. 29, 2010, each which are incorporated by reference herein in their entirety. User interface 128 may display information about flow cell 101 and the output signals received from chemical sensors in the sensor array on integrated circuit device 100. User interface 128 may also display instrument settings and controls, and allow a user to enter or set instrument settings and controls.

In an exemplary embodiment, during the experiment fluidics controller 118 may control delivery of individual reagents 114 to flow cell 101 and integrated circuit device 100 in a predetermined sequence, for predetermined durations, at predetermined flow rates. Array controller 124 can then collect and analyze the output signals of the chemical sensors indicating chemical reactions occurring in response to the delivery of reagents 114. During the experiment, the system may also monitor and control the temperature of integrated circuit device 100, so that reactions take place and measurements are made at a known predetermined temperature. The system may be configured to let a single fluid or reagent contact reference electrode 108 throughout an entire multi-step reaction during operation. Valve 112 may be shut to prevent any wash solution from flowing into passage 109 as reagents 114 are flowing. Although the flow of wash solution may be stopped, there may still be uninterrupted fluid and electrical communication between reference electrode 108, passage 109, and microwell array 107. The distance between reference electrode 108 and junction between passages 109 and 111 may be selected so that little or no amount of the reagents flowing in passage 109 and possibly diffusing into passage 111 reach reference electrode 108. In an exemplary embodiment, wash solution 110 may be selected as being in continuous contact with reference electrode 108, which may be especially useful for multi-step reactions using frequent wash steps.

FIG. 2 illustrates cross-sectional and expanded views of a portion of integrated circuit device 100 and flow cell 101. During operation, flow chamber 105 of flow cell 101 confines reagent flow 208 of delivered reagents across open ends of the reaction regions in microwell array 107. The volume, shape, aspect ratio (such as base width-to-well depth ratio), and other dimensional characteristics of the reaction regions may be selected based on the nature of the reaction taking place, as well as the reagents, byproducts, or labeling techniques (if any) that are employed. The chemical sensors of sensor array 205 are responsive to (and generate output signals to) chemical reactions within associated reaction regions in microwell array 107 to detect an analyte or reaction property of interest. The chemical sensors of sensor array 205 may for example be chemically sensitive field-effect transistors (chemFETs), such as ion-sensitive field effect transistors (ISFETs). Examples of chemical sensors and array configurations that may be used in embodiments are described in U.S. Patent Application Publication No. 2010/0300559, No. 2010/0197507, No. 2010/0301398, No. 2010/0300895, No. 2010/0137143, and No. 2009/0026082, and U.S. Pat. No. 7,575,865, each which are incorporated by reference herein in their entirety.

FIG. 3 illustrates a cross-sectional view of two representative chemical sensors and their corresponding reaction regions according to an exemplary embodiment. In FIG. 3, two chemical sensors 350, 351 are shown, representing a small portion of a sensor array that can include millions of chemical sensors. Chemical sensor 350 is coupled to corresponding reaction region 301, and chemical sensor 351 is coupled to corresponding reaction region 302. Chemical sensor 350 is representative of the chemical sensors in the sensor array. In the illustrated example, chemical sensor 350 is an ion-sensitive field effect transistor. Chemical sensor 350 includes floating gate structure 318 having a floating gate conductor (referred to herein as the sensor plate) separated from reaction region 301 by sensing material 316. As shown in FIG. 3, sensor plate 320 is the uppermost patterned layer of conductive material in floating gate structure 318 underlying reaction region 301.

In the illustrated example, floating gate structure 318 includes multiple patterned layers of conductive material within layers of dielectric material 319. The upper surface of sensing material 316 acts as sensing surface 317 for chemical sensor 350. In the illustrated embodiment, sensing material 316 is an ion-sensitive material, such that the presence of ions or other charged species in a solution in the reaction region 301 alters the surface potential of sensing surface 317. The change in the surface potential is due to the protonation or deprotonation of surface charge groups at the sensing surface caused by the ions present in the solution. The sensing material may be deposited using various techniques, or naturally formed during one or more of the manufacturing processes used to form chemical sensor 350. In some embodiments, sensing material 316 is a metal oxide, such as an oxide of silicon, tantalum, aluminum, lanthanum, titanium, zirconium, hafnium, tungsten, palladium, iridium, etc, or any other suitable metal oxide, or combination thereof In some embodiments, sensing material 316 is an oxide of the upper layer of conductive material of sensor plate 320. For example, the upper layer of sensor plate 320 may be titanium nitride, and sensing material 316 may comprise titanium oxide or titanium oxynitride. More generally, sensing material 316 may comprise one or more of a variety of different materials to facilitate sensitivity to particular ions. For example, silicon nitride or silicon oxynitride, as well as metal oxides such as silicon oxide, aluminum or tantalum oxides, generally provide sensitivity to hydrogen ions, whereas sensing materials comprising polyvinyl chloride containing valinomycin provide sensitivity to potassium ions. Materials sensitive to other ions such as sodium, silver, iron, bromine, iodine, calcium, and nitrate may also be used, depending upon the implementation.

The chemical sensor also includes source region 321 and drain region 322 within semiconductor substrate 354. Source region 321 and drain region 322 comprise doped semiconductor material have a conductivity type different from the conductivity type of substrate 354. For example, source region 321 and drain region 322 may comprise doped P-type semiconductor material, and the substrate may comprise doped N-type semiconductor material. Channel region 323 separates source region 321 from drain region 322. Floating gate structure 318 overlies channel region 323, and is separated from substrate 354 by gate dielectric 352. Gate dielectric 352 may be for example silicon dioxide. Alternatively, other suitable dielectrics may be used for gate dielectric 352. Reaction region 301 extends through fill material 310 on dielectric material 319. The fill material may for example comprise one or more layers of dielectric material, such as silicon dioxide or silicon nitride. Sensor plate 320, sensing material 316 and reaction region 301 may for example have circular cross-sections. Alternatively, these may be non-circular. For example, the cross-section may be square, rectangular, hexagonal, or irregularly shaped. The device in FIG. 3 can also include additional elements such as array lines (e.g. word lines, bit lines, etc.) for accessing the chemical sensors, additional doped regions in substrate 354, and other circuitry (e.g. access circuitry, bias circuitry etc.) used to operate the chemical sensors, depending upon the device and array configuration in which the chemical sensors described herein are implemented. In some embodiments, the device may for example be manufactured using techniques described in U.S. Patent Application Publication No. 2010/0300559, No. 2010/0197507, No. 2010/0301398, No. 2010/0300895, No. 2010/0137143, and No. 2009/0026082, and U.S. Pat. No. 7,575,865, each which are incorporated by reference herein in their entirety.

In operation, reactants, wash solutions, and other reagents may move in and out of reaction region 301 by diffusion mechanism 340. Chemical sensor 350 is responsive to (and generates an output signal related to) the amount of charge 324 present on sensing material 316 opposite sensor plate 320. Changes in charge 324 cause changes in the voltage on floating gate structure 318, which in turn changes in the threshold voltage of the transistor. This change in threshold voltage can be measured by measuring the current in channel region 323 between source region 321 and drain region 322. As a result, chemical sensor 350 can be used directly to provide a current-based output signal on an array line connected to source region 321 or drain region 322, or indirectly with additional circuitry to provide a voltage-based output signal. In an embodiment, reactions carried out in reaction region 301 can be analytical reactions to identify or determine characteristics or properties of an analyte of interest. Such reactions can generate directly or indirectly byproducts that affect the amount of charge adjacent to sensor plate 320. If such byproducts are produced in small amounts or rapidly decay or react with other constituents, multiple copies of the same analyte may be analyzed in reaction region 301 at the same time in order to increase the output signal generated. In an embodiment, multiple copies of an analyte may be attached to solid phase support 312, either before or after deposition into reaction region 301. The solid phase support may be microparticles, nanoparticles, beads, solid or porous gels, or the like. For simplicity and ease of explanation, solid phase support may also be referred herein as a particle. For a nucleic acid analyte, multiple, connected copies may be made by rolling circle amplification (RCA), exponential RCA, Recombinase Polymerase Amplification (RPA), Polymerase Chain Reaction amplification (PCR), emulsion PCR amplification, or like techniques, to produce an amplicon without the need of a solid support.

FIG. 4 illustrates a block diagram of an exemplary chemical sensor array coupled to an array controller, according to an exemplary embodiment. In various exemplary implementations, array controller 124 may be fabricated as a “stand alone” controller, or as a computer compatible “card” forming part of a computer 460, (See FIG. 8 in U.S. Pat. No. 7,948,015 for further details, which is incorporated by reference in its entirety herein). In one aspect, the functions of the array controller 124 may be controlled by computer 460 through an interface block 452 (e.g., serial interface, via USB port or PCI bus, Ethernet connection, etc.), as shown in FIG. 4. In one embodiment, array controller 124 is fabricated as a printed circuit board into which integrated circuit device 100 plugs; similar to a conventional IC chip (e.g., integrated circuit device 100 is configured as an ASIC that plugs into the array controller). In one aspect of such an embodiment, all or portions of array controller 124 may be implemented as a field programmable gate array (FPGA) configured to perform various array controller functions. For example, having determined an operational characteristic of sensors of the sensor array, the FPGA may be configured to select a group of sensors in the array based on the operational characteristic of sensors in the group and enable readout of the sensors in the selected group. Suitable readout circuitry may receive output signals from the enabled sensors, the output signals indicating chemical reactions occurring proximate to the sensors of the sensor array.

Generally, array controller 124 provides various supply voltages and bias voltages to integrated circuit device 100, as well as various signals relating to row and column selection, sampling of pixel outputs and data acquisition. In particular, array controller 124 reads the two analog output signals Vout1 (for example, odd columns) and Vout2 (for example, even columns) including multiplexed respective pixel voltage signals from integrated circuit device 100 and then digitizes these respective pixel signals to provide measurement data to computer 460, which in turn may store and/or process the data. In some implementations, array controller 124 also may be configured to perform or facilitate various array calibration and diagnostic functions, and an optional array UV irradiation treatment (See FIG. 11A in U.S. Pat. No. 7,948,015, which is incorporated by reference in its entirety herein, for further details). In general, the array controller provides the integrated circuit device with the analog supply voltage and ground (VDDA, VSSA), the digital supply voltage and ground (VDDD, VSSD), and the buffer output supply voltage and ground (VDDO, VSSO). In one exemplary implementation, each of the supply voltages VDDA, VDDD and VDDO is approximately 3.3 Volts.

As discussed above, in one aspect each of these power supply voltages is provided to integrated circuit device 100 via separate conducting paths to facilitate noise isolation. In another aspect, these supply voltages may originate from respective power supplies/regulators, or one or more of these supply voltages may originate from a common source in power supply 458 of array controller 124. Power supply 458 also may provide the various bias voltages required for array operation (e.g., VB1, VB2, VB3, VB4, VBO0, V_(BODY)) and the reference voltage VREF used for array diagnostics and calibration. In another aspect, power supply 458 includes one or more digital-to-analog converters (DACs) that may be controlled by computer 460 to allow any or all of the bias voltages, reference voltage, and supply voltages to be changed under software control (i.e., programmable bias settings). For example, power supply 458 responsive to computer control may facilitate adjustment of the bias voltages VB1 and VB2 for pixel drain current, VB3 for column bus drive, VB4 for column amplifier bandwidth, and VBO0 for column output buffer current drive. In some aspects, one or more bias voltages may be adjusted to optimize settling times of signals from enabled pixels. Additionally, the common body voltage V_(BODY) for all ISFETs of the array may be grounded during an optional post-fabrication UV irradiation treatment to reduce trapped charge, and then coupled to a higher voltage (e.g., VDDA) during diagnostic analysis, calibration, and normal operation of the array for measurement/data acquisition. Likewise, the reference voltage VREF may be varied to facilitate a variety of diagnostic and calibration functions. Reference electrode 108 which is typically employed in connection with an analyte solution to be measured by integrated circuit device 100 (See FIG. 1 in U.S. Pat. No. 7,948,015, which is incorporated by reference in its entirety herein, for further details), may be coupled to power supply 458 to provide a reference potential for the pixel output voltages. For example, in one implementation reference electrode 108 may be coupled to a supply ground (e.g., the analog ground VSSA) to provide a reference for the pixel output voltages based on Eq. (3) in U.S. Pat. No. 7,948,015. In one exemplary implementation, the reference electrode voltage may be set by placing a solution/sample of interest having a known pH level in proximity to integrated circuit device 100 and adjusting the reference electrode voltage until the array output signals Vout1 and Vout2 provide pixel voltages at a desired reference level, from which subsequent changes in pixel voltages reflect local changes in pH with respect to the known reference pH level. In general, it should be appreciated that a voltage associated with reference electrode 108 need not necessarily be identical to the reference voltage VREF discussed in U.S. Pat. No. 7,948,015 (which may be employed for a variety of array diagnostic and calibration functions), although in some implementations the reference voltage VREF provided by power supply 458 may be used to set the voltage of reference electrode 108.

Regarding data acquisition from integrated circuit device 100, in one embodiment array controller 124 of FIG. 4 may include one or more preamplifiers 253 to further buffer the output signals Vout1 and Vout2 from the sensor array and provide selectable gain. In one implementation, array controller 124 may include one preamplifier for each output signal (e.g., two preamplifiers for two analog output signals). In other aspects, the preamplifiers may be configured to accept input voltages from 0.0 to 3.3 Volts or from 0.1 to 5.0 Volts, may have programmable/computer selectable gains (e.g., 1, 2, 5, 10 and 20) and low noise outputs (e.g., <10 nV/sqrtHz), and may provide low pass filtering (e.g., bandwidths of 5 MHz and 25 MHz). In yet another implementation, the preamplifiers may have a programmable/computer selectable offset for input and/or output voltage signals to set a nominal level for either to a desired range. The array controller 124 may also comprise one or more analog-to-digital converters 454 (ADCs) to convert the sensor array output signals Vout1 and Vout2 to digital outputs (e.g., 10-bit or 12-bit) so as to provide data to computer 460. In one aspect, one ADC may be employed for each analog output of the integrated circuit device, and each ADC may be coupled to the output of a corresponding preamplifier (if preamplifiers are employed in a given implementation). In another aspect, the ADC(s) may have a computer-selectable input value ranging from 50 mV to 1 Volt, for example (e.g., 50 mV, 200 mV, 500 mV, 1 V) to facilitate compatibility with different ranges of array output signals and/or preamplifier parameters. In yet other aspects, the bandwidth of the ADC(s) may be greater than 60 MHz, and the data acquisition/conversion rate greater than 25 MHz (e.g., as high as 100 MHz or greater). ADC acquisition timing and array row and column selection may be controlled by timing generator 456. In particular, the timing generator provides the digital vertical data and clock signals (DV, CV) to control row selection, the digital horizontal data and clock signals (DH, CH) to control column selection, and the column sample and hold signal COL SH to sample respective pixel voltages for an enabled row. (See FIG. 9 in U.S. Pat. No. 7,948,015, which is incorporated by reference in its entirety herein, for further details). In one implementation, timing generator 456 may be implemented by a microprocessor executing code and configured as a multi-channel digital pattern generator to provide appropriately timed control signals. For example, timing generator 456 may be implemented as a field-programmable gate array (FPGA). For further details of row and column circuitry, see U.S. Pat. No. 7,948,015, which is incorporated by reference in its entirety herein.

FIG. 5 illustrates a method 500 according to an exemplary embodiment of how various operational characteristics of sensors of a sensor array may be taken into consideration during a read operation of the sensor array. In step 501, at least one operational characteristic of an individual sensor, a group of sensors, or all sensors of a sensor array is determined. Examples of operational characteristics of sensors include, but are not limited to, bead loading quality, a noise spectrum, and a threshold voltage value, and any combinations thereof In step 503, a group of sensors in the array based on the operational characteristic of sensors in the group may be selected. The selecting a group of sensors in the array may be based on more than one operational characteristic of sensors in the group. In step 505, readout of the sensors in the selected group may be enabled. According to exemplary embodiments, readout of remaining sensors of the sensor array may be bypassed. In step 507, output signals from the enabled sensors may be received, the output signals indicating chemical reactions occurring proximate to the sensors of the sensor array. Sensors in the sensor array may include chemically-sensitive field effect transistors. The chemically-sensitive field effect transistors may be arranged in rows and columns and the selecting includes selecting contiguous rows of chemically-sensitive field effect transistors in the sensor array. During an experiment, a fluidics controller may deliver individual reagents to the flow cell and integrated circuit device in a predetermined sequence. The output signals may indicate an ion concentration due to sequencing reactions occurring proximate to the chemically-sensitive field effect transistors. In an exemplary implementation, the output signals may be analog signals and the method may further include converting the output signals into digital signals and the receiving output signals may further include receiving the converted digital signals.

FIG. 6 illustrates a method 600 for nucleic acid sequencing according to an exemplary implementation. In step 601, template nucleic acids may be provided to at least some of a plurality of locations coupled to sensors of an array. In step 603, output signals of the sensors of the array may be analyzed to identify which locations in the plurality of locations contain the disposed template nucleic acids. In step 605, a group of sensors coupled to the identified locations containing the disposed template nucleic acids may be selected. In step 607, known nucleotides within at least some of the plurality of locations may be introduced. In step 609, the output signals of the selected sensors may be measured to detect sequencing reaction byproducts resulting from incorporation of the introduced known nucleotides into one of more primers hybridized to at least one of the disposed template nucleic acids. The sequencing reaction byproducts may comprise, for example, hydrogen ions, hydroxide ions, other ions, inorganic pyrophosphates (PPi), or any other suitable reaction byproduct or combination thereof. The sequencing reaction byproducts resulting from incorporation may be of chemically similar composition for each of the known nucleotides and sensors in the array detect a same byproduct. In step 611, readout of the sensors in the selected group, and bypassing readout of remaining sensors of the sensor array may be enabled. In step 613, at least a portion of sequences of at least a portion of the template nucleic acids may be determined based on the introduced known nucleotides and further based on the measured output signals. The sensors may comprise field-effect transistors having a chemically sensitive portion responsive to the sequencing reaction byproducts and may be disposed in proximity to the locations such that the at least one of the sequencing reaction byproducts diffuse or contact the sensors to thereby be detected. The chemically sensitive portion of the field-effect transistors of the array is responsive to a plurality of different sequencing reaction byproducts. The locations may be within respective reaction chambers. The measured output signals may be analog signals and the method may further include converting the output signals into digital signals and the receiving output signals may further include receiving the converted digital signals.

FIG. 7 illustrates examples of two different groups of sensors in an array on integrated circuit device 100 that have been selected based on an operational characteristic of sensors in the group. The sensors in the array illustrated in FIG. 7 are arranged in rows and columns. For example, a first group of sensors (defined by sensors within area 701) may be selected based on one (or more) operational characteristics of sensors in the group. Another group of sensors (defined by sensors within area 702) may be selected based on (a) different operational characteristic(s) of sensors in the group. The sensors within the wells may comprise fluid-addressable wells 705, and may also comprise reference wells 707. Thus, the group of sensors that is selected may be coupled to only fluid-addressable wells 705, only reference wells 707, or both fluid-addressable wells 705 and reference wells 707. In some embodiments, two or more, non-overlapping groups or partially-overlapping groups may be selected based on the same or different operational characteristics of sensors in the respective groups. Sensors in the two or more areas may be read out separately. First, sensors within area 701 may be read out, followed by sensors within area 702, or vice versa. Sensors within area 701 and 702 may be read out at the same time, while maintaining correspondence between the output signals and their respective sensors within a defined area (701/702, for example). The output signals from two or more corresponding areas may be compared with one another to determine which area provides an improved signal based on location of sensors on the array and/or based on the same or different operational characteristics of the sensors. The comparison may be used to predict high performance/preferred areas (sensors/sensor locations) for future experiments on unused integrated circuits/sensor arrays. The group(s) of sensors, operational characteristic(s), and the addressable area on the array may be dynamically selectable during an experiment or they may be predetermined before an experiment. The number of sensors in the group selected may vary, and the shape of the area defined by selected sensors may vary.

Embodiments of the above-described system provide particular technical advantages including an improvement in signal to noise ratio, and taking advantage of various operational characteristics of sensors of a sensor array, further enabling oversampling and improved speed in readout of output signals. Note that not all of the activities described above in the general description or the examples are required, that a portion of a specific activity may not be required, and that one or more further activities may be performed in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed. In the foregoing specification, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. After reading the specification, skilled artisans will appreciate that certain features are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range. While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims. 

1. A method comprising: determining an operational characteristic of each sensor of a plurality of sensors located at a portion of a sensor array, the determining occurring prior to introducing known nucleotides to at least some of the plurality of sensors; selecting a first group of sensors from the plurality of sensors located at the portion of the sensor array, wherein the selecting is based on determining that each sensor in the first group of sensors has a similar operational characteristic; enabling readout of the sensors in the first group of sensors; bypassing readout of other sensors of the sensor array that are not included in the first group of sensors, wherein the other sensors have a different operational characteristic than the first group of sensors; and receiving output signals from the first group of sensors, the output signals received after introducing known nucleotides to the at least some of the plurality sensors located at the portion of the sensor array.
 2. The method of claim 1, wherein the operational characteristic is a bead loading quality of the sensors of the sensor array.
 3. The method of claim 1, wherein the operational characteristic is a noise spectrum of the sensors of the sensor array.
 4. The method of claim 1, wherein the sensors are arranged in rows and columns and the selecting includes selecting sensors disposed in contiguous rows of the sensor array.
 5. The method of claim 1, wherein the output signals further indicate an incorporation of the introduced known nucleotides into primers hybridized to template nucleic acids disposed proximate to the sensors.
 6. The method of claim 1, wherein the output signals are analog signals and the method further includes converting the output signals into digital signals and the receiving output signals further includes receiving the converted digital signals.
 7. A method for nucleic acid sequencing, comprising: providing template nucleic acids to at least some of a plurality of locations coupled to sensors of an array; analyzing output signals of the sensors of the array to identify which locations in the plurality of locations contain the template nucleic acids and which locations in the plurality of locations do not contain the template nucleic acids; selecting a first group of sensors from the sensors in the array, wherein sensors in the first group of sensors are selected based on the locations containing the template nucleic acids; introducing known nucleotides within at least some of the plurality of locations to cause a reaction between the template nucleic acids and the known nucleotides; enabling readout of the sensors in the first group of sensors in the sensor array including the locations containing the template nucleic acids, and bypassing readout of other sensors of the sensor array, wherein the other sensors are in the locations not containing the template nucleic acids; and measuring the output signals of the first group of sensors to detect incorporation of the introduced known nucleotides into primers hybridized to the template nucleic acids, wherein the first group of sensors is selected prior to the reaction between the template nucleic acids and the known nucleotides.
 8. The method of claim 7, further comprising determining at least a portion of sequences of at least a portion of the template nucleic acids based on the introduced known nucleotides and further based on the measured output signals.
 9. The method of claim 7, wherein the locations are within respective reaction chambers.
 10. The method of claim 7, wherein the measured output signals are analog signals and the method further includes converting the measured output signals into digital signals.
 11. A system comprising: a sensor array including a plurality of sensors; and a processor, communicatively coupled to the sensor array, the processor configured to: determine an operational characteristic of each sensor of a plurality of sensors located at a portion of the sensor array before a chemical reaction occurs proximal to each sensor of prior to introducing known nucleotides to at least some of the plurality of sensors, select a first group of sensors from the plurality of sensors located at the portion of the sensor array based on determining that each sensor in the first group of sensors has a similar operational characteristic, enable readout of the sensors in the first group, bypass readout of other sensors of the sensor array that are not included in the first group of sensors, wherein the other sensors have a different operational characteristic from the first group of sensors, and receive output signals at the processor from the first group of sensors, the output signals received after introducing known nucleotides to the at least some of the plurality sensors located at the portion of the sensor array.
 12. The system of claim 11, wherein the operational characteristic is a bead loading quality of the sensors of the sensor array.
 13. The system of claim 11, wherein the operational characteristic is a noise spectrum of the sensors of the sensor array.
 14. The system of claim 11, wherein the sensors are arranged in rows and columns and the selecting includes selecting sensors disposed in contiguous rows of the sensor array.
 15. The system of claim 11, wherein the output signals further indicate an incorporation of the introduced known nucleotides into primers hybridized to template nucleic acids disposed proximate to the sensors.
 16. The system of claim 11, further comprising an analog-to-digital converter configured to convert analog output signals from sensors of the sensor array into digital output signals, wherein the processor is further configured to receive the digital output signals.
 17. The system of claim 11, wherein the processor is implemented in a field programmable gate array (FPGA). 